Buttler played a key role with the bat in a crucial 20-run win over New Zealand at the Gabba, which propelled England into the top two of Group 1 with one round of matches left.
The Black Caps, last year’s beaten finalists, still top the group but they, England and Australia are all now on five points.
Hosts and holders Australia are third and in real danger because of their vastly inferior net run rate. Only the top two sides go through to the semi-finals.
New Zealand play Ireland and Australia face already eliminated Afghanistan on Friday as the fight for the last four goes down to the wire.
England, who along with the hosts were the pre-tournament favourites, clash with Asian champions Sri Lanka the next day.
Buttler, who smashed 73 off 47 balls to lead England to 179-6 in Brisbane, said their clinical performance made up for a shock defeat earlier in the Super 12 stage to Ireland.
“Really proud of the guys tonight, to come out with freedom and express ourselves with our talent, I was really proud of the character we showed,” said the captain.
Buttler, who won the toss and chose to bat against a side who could have sealed a spot in the last four with victory, added: “We’re a really dangerous team, we’ve got match-winners throughout.
“Having not played our best (until now), we were desperate to show that today.” Buttler said it was to England’s advantage that they play the day after Australia in the fight for a semi-final spot.
“There’s a bit of a benefit playing that last game to know exactly what’s required of us. We’re going to expect a really tough game against Sri Lanka. “But we’ll go to Sydney full of confidence,” Buttler added.
Glenn Phillips led the Black Caps fightback in their reply, but once he fell for 62, the New Zealand challenge dissolved.
The Kiwis were left to rue two big dropped chances when danger man Buttler was at the crease.
“I guess they never help, he’s a very, very good player,” said all-rounder Mitch Santner of the failed attempts to get Buttler out.
“We knew the chase was going to be a challenge but I think we put ourselves in a pretty good position to get there, we just fell a bit short.”
‘All guns blazing’
In the first game of the day in Brisbane, Dhananjaya de Silva scored an unbeaten 66 as Sri Lanka kept their slim World Cup hopes alive with a six-wicket win over Afghanistan.
Sri Lanka are fourth in the table, a point behind the top three. They will need to beat England and hope other results go their way.
Assistant coach Naveed Nawaz said it would be “all guns blazing” for his side.
“I think we’ll have to go in a very positive mindset, thinking whatever the chance that we have to get in there, explore it,” he said.
“And whatever this risk that we have to take to get into the semis, we’ve got to take it.” Afghanistan are bottom of the group and cannot progress into the knockout stages.
This was their second defeat, with two other matches washed out by the rain.
“I’d say there’s been some good parts and there’s been some really average parts,” said Afghanistan’s coach Jonathan Trott, weighing up a campaign that has just never got going.
“We haven’t got things right,” the former England batsman said.
“We seem to be able to get ourselves into positions but then not utilise them and actually then kick on with the batting.” On Wednesday, in a doubleheader at Adelaide Oval, Zimbabwe face the winless Netherlands, and India play Bangladesh, both in Group 2.
With each team having two games to go, South Africa top the table with five points and are in the driving seat for a place in the semi-finals.
India and Bangladesh are second and third respectively, both on four points. Zimbabwe have three points.